Hi there - first thing - when you say this is a Gtechniq agent - is that confirmed from the Gtechniq website or is it just that the detailer says they are using Gtechniq - as like you say, I find it very surprising they would do it mobile. I’ve had the Gtechniq 7 year product done twice, and the dealership price is considerably more than going direct to the same detailer they use, privately. If its a new car, usually the dealer will allow your detailer to go to their stock room when the car arrives to do it prior to them handing the car over to you. Usually its the preparation of the car that takes the time and needs the indoor conditions, which is why a detailer would typically do it over 2 days. See Reepgroup.co.uk for example.
It is definitely worth it though IMO, will make the car much easier to clean and looking in showroom condition.
The ceramic coating industry is developing rapidly Now with a lot more products available, and more mobile retailers, and self service versions available. For me the real value is having the car cleaned/polished/repaired to the highest standard and then having it applied. You could do yourself if inclined, but for something that lasts 7 years, I think its worth paying a professional to do it.